Various criteria such as an “absolute maximum rating”, a “Direct Current (DC) standard”, or an “Alternate Current (AC) standard” to be satisfied are defined for an input signal to an electronic device such as a semiconductor device in terms of standards. The “absolute maximum rating” for an input signal is a voltage value that should not exceed even instantaneously, because the device may be destroyed at the voltage value. The “DC standard” for an input signal is a range of an input voltage within which the device can perform an intended normal operation. The “AC standard” for an input signal is a time standard such as an input timing of a signal, which should be satisfied for causing the device to perform an intended normal operation.
When a signal that violates the absolute maximum rating or the DC standard is input and output between electronic devices, the signal may cause a failure or an erroneous operation of an electronic device on the input side. In view of the above, a measure in which an appropriate termination resistance is disposed is required to be taken so that an input signal satisfies the absolute maximum rating or the DC standard. The value of the termination resistance in this case (hereinafter, referred to as a “termination resistance value”) is required to be an appropriate value, which is set so that an input signal satisfies the absolute maximum rating or the DC standard.
Meanwhile, a termination resistance may be incorporated in an electronic device in order to reduce a mounting area or the cost. For instance, a Double Data Rate 2-Synchronous Dynamic Random Access Memory (DDR2-SDRAM) is capable of shaping the waveform of an input signal with use of a built-in termination resistance, as an On Die Termination (ODT) function. Note that it is necessary for the user to set the termination resistance value to an appropriate value in advance.
As a method for obtaining an appropriate termination resistance value, there is a method by theoretical calculation such as simulation. In the method, it is necessary to consider various parameters such as drawing a wiring pattern, which may affect wiring electrical characteristics. However, it is not easy to obtain accurate values of parameters. Therefore, it is often the case that a large workload or a large amount of money is necessary for calculating an appropriate termination resistance value as a result.
Also, a method in which changing a termination resistance value and evaluating an actual device are repeated may be used in order to obtain an appropriate termination resistance value. In this method, it is necessary to evaluate an actual device each time a termination resistance value is changed. Therefore, an appropriate termination resistance value may not be obtained within a short period of time, and it is often the case that a large workload is required.
Various techniques for changing the termination resistance value of an input terminal are known. A multiprocessor device described in PTL 1 changes the termination resistance value according to the number of devices connected to a bus on the signal output side.
A memory control device described in PTL 2 detects occurrence of an undershoot or an overshoot. Further, the memory control device described in PTL 2 changes the termination resistance value when an undershoot or an overshoot occurs, and controls the driving performance of a device on the transmission side when it is difficult to eliminate an undershoot or an overshoot only by using the termination resistance value.